1. Field of the Invention
The present invention relates to a corrugated structure, such as a hose, a corrugated sheet or a set of louvers, which does not generate pure tones of noise, or whistles, when a fluid flows relative to the structure.
2. Description of the Related Art
In an ordinary corrugated structure, the distance between adjacent corrugations (pitch) is constant along the structure. The pitch of such known corrugated structures is purposely designed to be constant in order to aid in manufacturing and to provide appropriate stiffness of the structure.
However, constant-pitch structures have a propensity to produce pure tones, or whistles, when a fluid flows in a direction such that there is a velocity component perpendicular to the corrugations. Whistles are extremely annoying, particularly when they occur in the human hearing range (20-20,000 Hz). The frequency of a pure tone produced by fluid flow relative to a corrugated structure is a function of the mean flow velocity of the fluid and the constant pitch of the corrugations, and follows the relationship: EQU f=S(V/d) (1)
where:
f=frequency of a pure tone, in Hertz (1/second) PA1 S=Strouhal Number (dimensionless) PA1 V=mean flow velocity (subsonic), in feet/second PA1 d=pitch, or characteristic dimension, of the corrugations, in feet
The Strouhal Number, which is about 0.2 for subsonic flow, is empirically determined. See Engineering Noise Control, D. A. Bies & C. H. Hansen, Unwin Hyman Ltd., London (1988), pp. 322-324.
It has been observed that whistles produced by fluid flow relative to a corrugated structure occur over a wide range of flow velocities. Accordingly, a need exists for acoustically inactive (whistle-free) corrugated structures which are easy and inexpensive to manufacture.